Various techniques exist for measuring visual acuity which is typically represented by a "Snellen Equivalent," for example, 20/20, 20/25, 20/30, etc. (or 6/6, 6/8, 6/9 using the metric system). Measuring visual acuity is typically based on the size of a letter or symbol being viewed and the distance from the letter or symbol at which the person being tested can see or visually recognize the letter or symbol. According to the "Snellen Equivalent," the numerator or first number "20" (or "6") represents a 20 foot (or 6 meter) distance from the person being tested to the target, such as an eye chart. The denominator or second number refers to the size of the letter or symbol being viewed. For example, the 20/40 (or 6/12) letter on an eye chart is twice the size of the 20/20 (or 6/6) letter on the eye chart i.e. 18 mm vs. 9 mm. If the person being tested can see the 20 (or 6) size letter at 20 feet (or 6 meters) from the eye chart, that person is said to have 20/20 (or 6/6) vision.
The primary difficulty with using this standard for measuring visual acuity is the 20 foot distance limitation. Many eye examination rooms are not large enough to position the person being tested at 20 feet from the target or eye chart. Often, a mirror and projector system must be used to simulate a distance of 20 feet. Other expensive computerized systems can also be programmed for distances other than 20 feet. Even if such systems are used, the users often do not accurately measure the distances, resulting in inaccurate visual acuity measurements.
Another problem with traditional visual acuity measuring systems occurs when a person cannot see the largest letter or symbol on the chart. This is common when someone who wears glasses is being tested without their glasses. In these cases, the person being tested must often move toward the chart until they can read the letter or symbol and their distance from the chart must then be estimated. Other approaches to this problem have been to provide a small cube with a Snellen letter (e.g. E) and to move the letter toward the person being tested until the letter can be read. The distance from the letter to the person is then estimated, and the visual acuity is calculated, i.e. the "Snellen Equivalent."
The estimation of the distance between the person being tested and the eye chart or letter is typically crude and inaccurate. The resulting "Snellen Equivalent" calculated from the estimated distance is thus often an inaccurate measurement of visual acuity.
Accordingly, what is needed is a portable visual acuity testing system and method that allows visual acuity to be accurately and automatically tested without having to manually estimate distances or make calculations. The portable visual acuity testing system and method should avoid having to position the person being tested at a fixed distance from the testing device, and should avoid having to simulate the 20 foot distance with projectors or computers in an examination room that is too small. Moreover, the portable visual acuity testing system and method should provide the added flexibility of varying the sizes and types of the letters or symbols.